1. Field of the Invention
This invention relates generally to a semiconductor device and particularly to a four region light activated silicon switch (LASS).
2. Description of the Prior Art
In order to actuate a LASS, a large amount of light must be delivered to the proper regions within the wafer of semiconductor material comprising the semiconductor device. To accomplish this it has been the practice to direct a beam of light on to at least one surface of the silicon wafer. Typically, the beam is sent in a perpendicular direction to the wafer surface or at Brewster's angle in order to minimize the surface reflection loss.
There are some rather severe disadvantages with these prior art techniques for light coupling to the LASS. Typically, the light is introduced through a central aperture, or one or more rectangular slots arranged in the cathode electrode. Thus the light is introduced into a relatively compact area. It is characteristic of the LASS that current conduction spreads slowly from the region initially turned on as determined by the area exposed to light. Thus, it takes a relatively long time for conduction to spread from the turned on area to cover the entire useful area of the device.
From the illuminated or turned on area, the shortest electrical path is in a direction toward the aperture in the cathode electrode, but obviously, since the aperture presents no electrical contact, the path to the cathode electrode must be longer. Thus much of the initial current passes through long, relatively high resistance paths in the silicon before it can enter the metal cathode electrode or contact surrounding the aperture. This high resistance causes rapid heating at the beginning of current flow, and limits the amount of current that can be tolerated without device failure.
Further, with the single aperture, or the one or more rectangular slots in the cathode electrode, the heat generated in the silicon wafer in the area where the light impinges must seek a longer thermal path to the cathode electrode surrounding in the aperture. There is of course still a thermal path to an anode electrode which also serves as a heat sink. As a result, thermal considerations place an upper limit on the starting current capacity for the device.
A typical prior art device is described in British Pat. No. 1,254,634 for "Improved Thyristor Arrangement" invented by Boksjo et al. In this patent the thyristor is turned on by a controlled illumination which is applied normal to the semiconductor body through windows arranged in both the anode and cathode sides of the LASS.